This invention relates to rectifiers and more particularly to Schottky barrier rectifying devices, and methods of forming these devices.
Rectifiers exhibit relatively low resistance to current flow in a forward direction and a high resistance to current flow in a reverse direction. Schottky barrier rectifiers are a type of rectifier that have found use as output rectifiers in switching-mode power supplies and in other high-speed power switching applications, such as motor drives. These devices are capable of carrying large forward currents and supporting large reverse blocking voltages.
U.S. Pat. No. 5,365,102 to Mehrotra et al. and entitled xe2x80x9cSchottky Barrier Rectifier with MOS Trenchxe2x80x9d, the entire disclosure of which is hereby incorporated by reference, discloses Schottky barrier rectifiers which have a higher breakdown voltage than is theoretically attainable with an ideal abrupt parallel-plane P-N junction. A cross-sectional representation of one embodiment of the described rectifiers is illustrated in FIG. 1. In this figure, rectifier 10 includes a semiconductor substrate 12 of first conductivity type, typically N-type conductivity, having a first face 12a and a second opposing face 12b. The substrate 12 comprises a relatively highly doped cathode region 12c (shown as N+) adjacent the first face 12a. A drift region 12d of first conductivity type (shown as N) extends from the cathode region 12c to the second face 12b. Accordingly, the doping concentration of the cathode region 12c is greater than that of the drift region 12d. A mesa 14 having a cross-sectional width xe2x80x9cWmxe2x80x9d, defined by opposing sides 14a and 14b, is formed in the drift region 12d. The mesa can be of stripe, rectangular, cylindrical or other similar geometry. Insulating regions 16a and 16b (described as SiO2) are also provided on the mesa sides. The rectifier also includes an anode electrode 18 on the insulating regions 16a, 16b. The anode electrode 18 forms a Schottky rectifying contact with the mesa 14 at second face 12b. The height of the Schottky barrier formed at the anode electrode/mesa interface is dependent on the type of electrode metal and semiconductor (e.g., Si, Ge, GaAs, and SiC) used and is also dependent on the doping concentration in the mesa 14. Finally, a cathode electrode 20 is provided adjacent the cathode region 12c at the first face 12a. The cathode electrode 20 ohmically contacts cathode region 12c. 
As the voltages of modern power supplies continue to decrease in response to the need for reduced power consumption and increased energy efficiency, it becomes more advantageous to decrease the on-state voltage drop across a power rectifier, while still maintaining high forward-biased current density levels. As well known to those skilled in the art, the on-state voltage drop is generally dependent on the forward voltage drop across the metal/semiconductor junction and the series resistance of the semiconductor region and cathode contact.
The need for reduced power consumption also generally requires minimizing the reverse-biased leakage current. The reverse-biased leakage current is the current in the rectifier during a reverse-biased blocking mode of operation. To sustain high reverse-biased blocking voltages and minimize reverse-biased leakage currents, the semiconductor portion of the rectifier is typically lightly doped and made relatively thick so that the reverse-biased electric field at the metal/semiconductor interface does not become excessive. The magnitude of the reverse-biased leakage current for a given reverse-biased voltage is also inversely dependent on the Schottky barrier height (potential barrier) between the metal and semiconductor regions. Accordingly, to achieve reduced power consumption, both the forward-biased voltage drop and reverse-biased leakage current should be minimized and the reverse blocking voltage should be maximized.
According to U.S. Pat. No. 5,612,567, desirable effects are achieved with the device of FIG. 1, due to the occurrence of charge coupling between the majority charge carriers in the mesa-shaped portion of the drift region 14 and the portion of the metal anode 18 opposite the insulated sidewalls 16a, 16b of the trenches. Specifically, the electric field at the center of the metal-semiconductor contact (Schottky contact) is reduced significantly relative to an ideal plane-parallel rectifier. The reduction in electric field at the center of the Schottky contact causes a significant decrease in the reverse-biased leakage current through a reduction in Schottky barrier height lowering. Reverse-biased leakage current is the current in the rectifier during a reverse-biased (blocking) mode of operation. Moreover, the peak in the electric field profile shifts away from the metal-semiconductor contact and into the drift region. As the peak of the electric field moves away from the Schottky contact, the mesa is able to support more voltage, and thus provides higher breakdown voltages (reverse blocking voltages) than those of an ideal parallel-plane rectifier.
A graphical illustration of the breakdown voltage versus trench oxide thickness for the Schottky rectifier shown in FIG. 1 is illustrated in FIG. 2, which is a reproduction of FIG. 12 of the aforementioned patent. In particular, the breakdown voltage is shown as monotonically increasing with oxide thickness up to at least 2200 Angstroms. The graphical illustration of FIG. 2 was obtained for a Schottky rectifier having a mesa width and cell pitch of 0.5 microns and 1 micron, respectively, and a trench depth and drift region thickness of 3 microns and 4 microns, respectively.
As FIG. 2 indicates, Schottky rectifiers to be used in high voltage applications require a relatively thick trench oxide layer. The oxide layer is typically grown by a thermal technique, which is advantageous employed because it provides good epitaxy with a reduced defect density at the oxide-semiconductor interface. Unfortunately, the slow growth rates associated with thermally grown oxide layers make it difficult to achieve a trench oxide layer with a thickness upwards of 2000 Angstroms. Moreover, alternative growth techniques such as chemical vapor deposition (CVD), while having greater deposition rates, produce a greater defect density and hence a higher charge at the oxide-semiconductor interface.
Accordingly, there remains a need within the art to provide a trench Schottky rectifier device that can be operated at high voltages and which is relatively easy to fabricate.
The above and other needs are met by the present invention. In particular, a Schottky rectifier is provided which comprises: (a) a semiconductor region having first and second opposing faces, with the semiconductor region comprising a cathode region of first conductivity type adjacent the first face and a drift region of the first conductivity type adjacent the second face, and with the drift region having a lower net doping concentration than that of the cathode region; (b) one or more trenches extending from the second face into the semiconductor region and defining one or more mesas within the semiconductor region; (c) an insulating region adjacent the semiconductor region in lower portions of the trench; (d) and an anode electrode that is (i) adjacent to and forms a Schottky rectifying contact with the semiconductor region at the second face, (ii) adjacent to and forms a Schottky rectifying contact with the semiconductor region within upper portions of the trench and (iii) adjacent to the insulating region within the lower portions of the trench.
Preferably, the semiconductor is silicon, the first conductivity type is n-type conductivity, and a cathode electrode is provided on the first face.
The lower portions of the trenches preferably correspond to approximately 25 to 40% of the depth of the trenches. In some embodiments, the trench extends into the cathode region, with the insulated lower portions of the trench preferably extending between the cathode region and the drift region.
The insulating region preferably comprises silicon dioxide, which can be either deposited or thermally grown.
In some embodiments, a polysilicon region is disposed on the insulating region and forms part of the anode electrode.
The present invention also provides a method of forming a trench Schottky rectifier. The method comprises: (a) forming a semiconductor region having first and second opposing faces, with the semiconductor region comprising a cathode region of first conductivity type adjacent the first face and a drift region of the first conductivity type adjacent the second face, and with the drift region having a lower net doping concentration than that of the cathode region; (b) forming one or more trenches extending from the second face into the semiconductor region, with the trenches defining one or more mesas within the semiconductor region; (c) forming an insulating region adjacent the semiconductor region in lower portions of the trench; (d) and forming an anode electrode that is (i) adjacent to and forms a Schottky rectifying contact with the semiconductor region at the second face, (ii) adjacent to and forms a Schottky rectifying contact with the semiconductor region within upper portions of the trench and (iii) adjacent to the insulating region within the lower portions of the trench.
The step of forming the semiconductor region preferably comprises providing a semiconductor substrate that corresponds to the cathode region, and growing an epitaxial semiconductor layer that corresponds to the drift region on the substrate.
The step of forming the trenches preferably comprises the steps of forming a patterned masking layer over the second face of the semiconductor region and etching the trenches through the masking layer.
The step of forming the insulating region can comprise providing an oxide layer over the second face and in the trenches, and subsequently etching portions of the oxide layer. In some embodiments, a photoresist pattern is provided on the oxide layer (which can be thermally grown), and portions of the oxide layer not covered by the photoresist etched, whereupon the photoresist is removed. In other embodiments, a polysilicon layer is provided on the oxide layer (which can be thermally grown), and the polysilicon layer is etched such that portions of the oxide layer over the second face and over the upper portions of the trenches are exposed, and these exposed portions are subsequently removed by etching.
The step of forming the insulating region can also comprise depositing an oxide layer. For example, a tetraethylorthosilicate layer can be deposited on the second face and within the trenches. The tetraethylorthosilicate layer can then be etched until it is removed from the second surface and the upper portions of the trenches. Subsequently, the tetraethylorthosilicate can be converted into a high-density silicon dioxide layer.
One advantage of the present invention is that a novel Schottky barrier rectifier is provided having low forward-biased voltage drop, low reverse-biased leakage current and high breakdown voltage.
Another advantage is that such a Schottky barrier rectifier can be made using simple, and thus economical, manufacturing techniques.
Still other embodiments and advantages will become readily apparent to those skilled in the art upon review of the Detailed Description, Examples and claims set forth below.